Method and device for producing ball-shaped metallic particles at least almost equal in diameter, and the ball-shaped metallic particles made by the method

ABSTRACT

Method and device for producing ball-shaped metallic particles at least almost equal in diameter are disclosed comprising a cylindrical metallic housing, a vessel provided on the cylindrical metallic housing, the vessel having a plurality of small openings through a bottom plate thereof, a vibrator disposed on a mount to locate above the vessel in a manner that the vessel may be subjected to vibration, a pair of pipes provided to locate at an opposition portion of the cylindrical metallic housing to deliver and fill nitrogen or inert gas within the housing, an inclined bottom plate having a soft layer arranged at a bottom portion of the housing to form a shielding structure, an exhaust pipe provided outwardly at an outlet of the inclined bottom plate and a selector arranged at a bottom outlet of the shielding structure.  
     A number of approximately small ball-shaped metallic particles almost equal in diameter and having a luster on each surface can be manufactured continuously.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to method and device for producing small ball-shaped metallic particles at least almost equal in diameter, and the ball-shaped metallic particles made by the method, and more particularly to method and device for producing small ball-shaped metallic particles at least almost equal in diameter which can be used for soldering a base circuit of semiconductors.

[0003] A great many of small ball-shaped metallic particles are widely used for soldering or powder metallurgy, and a number of methods and devices for producing particles of this type from a molten metal are known, for example, particles of liquid metal are produced by means of dissecting liquid metal above a gas stream flowing against the force of gravity, the articles then adopting a ball shape due to the surface tension of the liquid metal and solidification.

[0004] In another method, a metallic wire or plate is cut into pieces, each having a given length, which are pressed and rolled into small metallic particles, or alternately the cut pieces are melted and the molten metal is rolled into small metallic particles.

[0005] The small ball-shaped metallic particles thus produced have the disadvantage that there is a difference of the melting temperatures of the small metallic particles made from the starting material made from an end portion and that of a middle portion thereof.

[0006] As a result, there is a tendency that when spontaneous soldering of semiconductor circuits is carried out at a determined temperature, some portions of the circuits are not soldered.

[0007] It is likely that some metallic particles are either deformed or their surfaces are damaged.

[0008] A good small ball-shaped metallic particle usually has a brilliant metallic luster on its surface so that the quality of the produced metallic particle can be judged by good or bad of the luster of the small ball-shaped metallic particle as its standard.

[0009] 2. Description of the Prior Art

[0010] According to the recent method described in the published specification of Japanese Patent Publication No.43794/1987 (not-examined), the molten metal is sprayed into the air, and as shown in the published specification of Japanese Patent Publication No.184201/1988, the method of producing the small ball-shaped metallic particles is shown, in which a molten stream of metal is flown under pressure through an opening of a container in order to produce the small ball-shaped metallic particles under the surface tension.

[0011] There is a tendency, however, that a lot of either deformed or damaged small ball-shaped metallic particles are produced to show a lower yield so that it is rather difficult to produce a number of the good small ball-shaped metallic particles efficiently.

[0012] As shown in the published specification of Japanese Patent Publication No.144216/2000 (not-examined), the liquid metal is discharged through an opening of a container to atomize in the air and to solidify into droplets by the surface tension of the liquid metal.

[0013] In accordance with this method, however, liquid metal particles are dropped into an oily solution which does not vaporize even at 200° C. in order to prevent collision of the dropping small particles with each other or to avoid a deformation by a shock on a hard bottom Plate.

[0014] This method, however, has the disadvantage that the metallic surfaces of the produced metallic particles are oxidized by the oily solution not to form an independent brilliant metallic luster on the surfaces of the produced small ball-shaped metallic particles.

SUMMARY OF THE INVENTION

[0015] A primary object of this invention is to provide a method for producing small ball-shaped metallic particles at least almost equal in diameter that can be used for soldering a base circuit of semiconductors.

[0016] Another object of this invention is to provide a device for producing small ball-shaped metallic particles at least almost equal in diameter that can be used for soldering a base circuit of semiconductors.

[0017] Another object of this invention is to provide a device for producing small ball-shaped metallic particles at least almost equal in diameter having a physically, structurally and homogeneously better property.

[0018] Another object of this invention is to provide a method for producing small ball-shaped metallic particles at least almost equal in diameter whereby a troublesome washing process for recovering the metallic particles in an oily solution can be abolished.

[0019] Another object of this invention is to provide a method for producing small ball-shaped metallic particles at least almost equal in diameter whereby troublesome disposal of waste oil can be abolished.

[0020] Another object of this invention is to provide a device, which is simple in construction, so that it can be made easily.

[0021] Another object of this invention is to provide a method for producing small ball-shaped metallic particles at least almost equal in diameter whereby the soft droplets drop on the soft inclined bottom plate; there appear no dents, flaws or deformation on the surface of the metallic particles by cushion of the soft inclined bottom plate.

[0022] Still another object of this invention is to provide a small ball-shaped metallic particle at least almost equal in diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] This object and features of the invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which:

[0024]FIG. 1 is a front elevation, partly in perspective, of a device for producing ball-shaped metallic particles at least almost equal in diameter;

[0025]FIG. 2 is a plan view of the device shown in FIG. 1;

[0026]FIG. 3 is an enlarged schematic longitudinal sectional view of a device for producing small ball-shaped metallic particles at least almost equal in diameter of this invention, taken on line 111-111 of FIG. 1; and

[0027]FIG. 4 is a flow-chart showing the steps of a method for producing small ball-shaped metallic particles at least almost equal in diameter of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Referring now to the drawings, especially FIGS. 1-3, a device 10 for producing small ball-shaped metallic particles at least almost equal in diameter comprises a cylindrical metallic housing 12 and a vessel 14 which is provided through a central opening 12 b to locate at a middle portion of the ceiling 12 a of the cylindrical metallic housing 12.

[0029] Provided through a bottom plate 14 a of the vessel 14 are a plurality of small openings 14 b which are so small that a liquid or molten metal supply 30 does not flow downwardly at a standstill condition by the surface tension of the liquid metal 30.

[0030] A vibrator 18 is disposed on a mount 16, which is arranged on the ceiling 12 a to locate above the vessel 14 in a manner that the vessel 14 may be subjected to vibration by the vibrator 18.

[0031] A pair of pipes 24, each having a valve 24 a, is provided to locate at a middle and opposition portions of the cylindrical metallic housing 12, thus enabling to deliver and fill nitrogen or inert gas G within the housing 12.

[0032] An inclined bottom plate 20 having a soft layer 20 a is arranged at a bottom portion of the housing 12 to form a shielding structure 22, and an exhaust pipe 26 is provided outwardly at an outlet of the inclined bottom plate 20.

[0033] A selector 26 a such as mesh, gravitation or wind selecting means is arranged at a bottom outlet 20 d of the exhaust pipe 26.

[0034] In another embodiment, either soft plate 20 a or cloth 20 b, each having a high igniting point, which does not burn even at a high temperature can be mounted on the inclined bottom plate 20.

[0035] For example, inorganic fiber such as rock wool, vegetable fiber or a soft plate made from pulp can be used for the soft plate 20 a.

[0036] Inorganic fiber that is made from a woven fabric or soft cloth of vegetable fiber is used for the soft cloth 20 b.

[0037] A composite or a plurality layer of the soft plates 20 a or the soft cloth 20 b can be preferably used, or alternatively a composite layer of the soft cloth 20 b having a hollow air layer can be used.

[0038] It should be understood that a number of approximately small ball-shaped metallic particles 30 almost equal in diameter and having a luster on each surface could be manufactured continuously by the method and device of this invention.

EXAMPLE 1

[0039] The liquid metal 30 is supplied into the vessel 14 having a plurality of the small openings 14 b that are so small that the liquid metal 30 is not allowed to flow downwardly through the openings 14 b at a standstill condition by the surface tension of the liquid metal 30.

[0040] A diameter of the opening 14 b of the bottom plate 14 a is small enough not to allow the liquid metal 30 flow downwardly through the opening 14 b.

[0041] When the vessel 14 is subjected to vibration by the vibrator 18, the liquid metal 30 flows downwardly through the small openings 14 b so that the segments of the dissected stream take a ball shape due to the surface tension of the liquid metal 30 into a number of the molten droplets 30 a in the shielding structure 22.

[0042] Further, the molten droplets 30 a are taking on a ball shape in free fall through the nitrogen or inert gas G surrounding them in the shielding structure 22.

[0043] The molten droplets 30 b are cooled and solidified while dropping, and the solidified droplets 30 c drop and roll on the soft plate 20 a or the soft cloth 20 b towards the outside of the inclined bottom plate 20.

[0044] The solidified spherical droplets 30 c rolling down on the soft plate 20 a or the soft cloth 20 b are delivered into a container 28, which is subsequently subjected to a selection by a known selecting device (not shown) to select a number of approximately small ball-shaped metallic particles 30 d almost equal in diameter and having a luster on each surface of the metallic particles 30 d.

EXAMPLE 2

[0045] The soft plate 20 a or the soft cloth 20 b, each having a high igniting point, is preferably used for the soft inclined bottom plate 20.

[0046] In accordance with the method and device of this invention, approximately small ball-shaped metallic particles 30 almost equal in diameter and having a luster on each surface can be manufactured continuously with a high yield.

EXAMPLE 3

[0047] A method for producing small ball-shaped metallic particles at least almost equal in diameter of this invention is explained with reference to an embodiment shown in FIGS. 3 and 4.

[0048] As in the foregoing embodiments, a plurality of small openings 14 b is provided at a given distance through the inclined bottom plate 20 so that the molten droplets 30 a cannot flow through the opening 14 b of the bottom plate 14 a at a standstill condition.

[0049] A diameter of the opening 14 b can be decided by a kind of the liquid metals 30, such as, tin, bismuth, zinc, aluminum etc.

[0050] When a diameter of the opening 14 b of the bottom plate 14 a is less than 0.2 mm, the liquid metal 30 does not flow through the openings 14 b by vibration only so that it is necessary to apply the additional pressure.

[0051] In this embodiment, the vessel 14 is subjected to vibration by the vibrator 18 to deliver the liquid metal 30 downwardly through the small openings 14 b into an atmosphere containing no oxygen or inert gas G comprising nitrogen or a mixture of nitrogen and another gas in the shielding structure 22.

[0052] While dropping, the molten droplets 30 a are taking on a ball shape in free fall through the nitrogen or inert gas G surrounding them in the shielding structure 22 by the surface tension. In this embodiment, no oxygen exists around the surface of each droplet 30 a so that it is not oxidized to have a metallic luster on the surface of the droplet 30 a.

[0053] During further dropping, the ball-shaped droplets 30 a are cooled into a certain degree and solidified.

[0054] Solidification of the dropping ball-shaped droplets 30 a depends on the temperature of the atmosphere in the housing 12, thus requiring a long cooling time along with a melting temperature and a given dropping height of the ball-shaped droplets 30 a.

[0055] For example, a soldering material is dissolved at a temperature between 183° C.-300° C., and the dissolved soldering material solidifies at room temperature at a height between 5 m˜6 m so that a height for dropping the metallic particles 30 a must be over 5 m˜6 m.

[0056] Aluminum is dissolved at a temperature around 600° C., and the dissolved aluminum solidifies at room temperature at a height between 45 m˜50 m so that a height for dropping the metallic particles must be over 50 m.

[0057] When liquefied nitrogen is used to make a nitrogen atmosphere in the shielding structure 22, the liquefied nitrogen itself is used as a cooling gas at vaporization.

[0058] When the dropping atmosphere can be cooled, a cooling solidification can be accelerated to shorten the dropping height.

[0059] The solidified small ball-shaped metallic particles 30 c drop on the soft inclined bottom plate 20, on which shock of the dropped ball-shaped metallic particles 30 c are softened and dispersed by a cushion in order to prevent dents and deformation of the surface of the ball-shaped metallic particles 30 c.

[0060] When the ball-shaped metallic particles 30 c hit directly and vertically the hard inclined bottom plate 20, some of the ball-shaped metallic particles 30 c are crushed or some plane portions thereof are formed on the spherical surfaces 30 c to destroy the small ball-shaped metallic particles at least almost equal in diameter and also to lose brilliant metallic luster on the surfaces of the ball-shaped metallic particles 30 c.

[0061] Subsequently, the ball-shaped metallic particles 30 c roll downwardly into the exhaust pipe 26.

[0062] As mentioned in the foregoing paragraphs, when the ball-shaped metallic particles 30 c hit directly and vertically the hard inclined bottom plate 20, the ball-shaped metallic particles 30 c are either deformed or hurt by the dropping shock to lose brilliant metallic luster on the surfaces of the ball-shaped metallic particles 30 c.

[0063] The ball-shaped metallic particles 30 c are delivered into the selecting device to select a number of approximately small ball-shaped metallic particles 30 d almost equal in diameter and having a luster on each surface.

EXPERIMENTAL EXAMPLE 1

[0064] The ball-shaped metallic particles 30 d having a mixture of 63% of tin and 37% of lead were dropped on the hard inclined bottom plate 20 through the opening 14 b of the bottom plate 14 a from a height of 4 m.

[0065] In case a diameter of the opening 14 b of the bottom plate 14 a is less than 0.65 mm, the small ball-shaped metallic particles 30 d almost equal in diameter and having no flaw, but having the luster on each surface can be made with a high yield.

[0066] In case a diameter of the small ball-shaped metallic particle 30 d is over 0.66 mm, there appear many flaws and few luster on each surface of the produced small ball-shaped metallic particle 30 d.

[0067] It has been confirmed by this experiment that collision of the small ball-shaped metallic particles 30 d deteriorate their quality, and that the smaller the diameter of the particle 30 d, the fewer the poor particles 30 d.

EXPERIMENTAL EXAMPLE 2

[0068] The ball-shaped metallic particles 30 d having a mixture of 10% of tin and 90% of lead were dropped on the hard inclined bottom plate 20 through the opening 14 b of the bottom plate 14 a from a height of 4 m.

[0069] In case a diameter of the small ball-shaped metallic particle 30 d is 0.8 mm, the solidified ball-shaped metallic particle 30 d is somewhat soft and there appear flaws on a surface of the 100% particles 30 d.

[0070] When the soft solidified small ball-shaped metallic particles 30 c drop on the soft inclined bottom plate 20, there appear no dents, flaws or deformation on the surface of the metallic particles 30 c by a cushion of the soft inclined bottom plate 20.

[0071] This invention provides the method and device for producing small ball-shaped metallic particles at least almost equal in diameter.

[0072] It is understood that modifications to the invention as described may be made, as might occur to one with skill in the field of the invention, within the scoped of the appended claims. All embodiments contemplated hereunder, which achieve the objects of the invention, have therefore not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims. 

We claim:
 1. A method for producing small ball-shaped metallic particles at least almost equal in diameter, which comprises the steps of: (a) supplying liquid metal into a vessel; (b) delivering downwardly said liquid metal through a plurality of small openings provided through a bottom plate of the vessel that are so small that said liquid metal is not allowed to flow at a standstill condition while applying vibration into the vessel; (c) introducing and cooling said molten droplets through the nitrogen or inert gas surrounding them in a shielding structure so that the segments of the dissected stream take a ball shape due to the surface tension of the molten droplets; (d) dropping and rolling said solidified molten droplets metal on an inclined bottom plate towards the outside of said inclined bottom plate, and; (e) delivering said solidified spherical droplets into a container, which is subsequently subjected to a selection by a known selecting device to select a number of approximately small ball-shaped metallic particles almost equal in diameter and having a luster on each surface of the metallic particles.
 2. A device for producing small ball-shaped metallic particles at least almost equal in diameter, which comprises: (a) a cylindrical metallic housing; (b) a vessel which is provided through a central opening to locate at a middle portion of a ceiling of said cylindrical metallic housing, said vessel having a plurality of small openings which are provided through a bottom plate of the vessel, said each opening being so small that a liquid metal does not flow downwardly at a standstill condition by the surface tension of the liquid metal; (c) a vibrator disposed on a mount, which is arranged on the ceiling to locate above the vessel in a manner that the vessel may be subjected to vibration by said vibrator; (d) a pair of pipes, each having a valve, provided to locate at an opposite portion of said cylindrical metallic housing, thus enabling to deliver and fill nitrogen or inert gas within said housing; (e) an inclined bottom plate having a soft layer being arranged at a bottom portion of said housing to form a shielding structure; (f) an exhaust pipe provided and outwardly at an outlet of said inclined bottom plate; and (g) a selector such as a mesh, gravitation or wind selecting means being arranged at a bottom outlet of the shielding structure.
 3. A device of claim 2 wherein either soft plate or cloth, each having a high igniting point, which does not burn even at a high temperature is mounted on the inclined bottom plate.
 4. A device of claim 2 wherein inorganic fiber such as rock wool, vegetable fiber or a soft plate made from pulp is used for the soft plate.
 5. A device of claim 4 wherein inorganic fiber such inorganic fiber that is made from a woven fabric or soft cloth of vegetable fiber is used for the soft cloth.
 6. A device of claim 5 wherein a composite or a plurality layer of the soft plates or the soft cloth is preferably used, or alternatively a composite layer of the soft cloth having a hollow air layer is provided.
 7. A device of claim 2 wherein the cylindrical metallic housing contains inert gas comprising nitrogen or a mixture of nitrogen and another gas in the shielding structure.
 8. A small ball-shaped metallic particle at least almost equal in diameter made by the method. 